The Biancocelesti had interrupted a dismal streak with a 2-0 win over Livorno last week, but it was back to the old problems at the Stadio Bentegodi.
"I saw a few difficulties, but we conceded the first two goals on set plays," Petkovic told Mediaset Premium.
"We dominated this game and created many chances, so it is absurd to lose like this. It's incomprehensible, as we played better than Verona.
"This season we created so much and took home so little. We've got to make more of our chances. We make too many mistakes both in attack and defence, so at the moment we are lacking in confidence."
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